As a multilevel modulation type modulator, a QAM modulator is known (refer to, for example, PTLs 1 and 2).
A modulator described in PTL 1 obtains a 16QAM signal by providing a loss section (6 dB) on one of output arm portions and superimposing quadri-phase shift keying (QPSK) signals in which an output difference therebetween is 6 dB. Further, a configuration is adopted in which an input-output branching section is formed as a planar optical waveguide (PLC) on a glass substrate and the glass substrate is optically connected to a lithium niobate substrate. On the other hand, also in an optical modulator described in PTL 2, an optical power ratio is adjusted to a quarter by providing an optical power adjustment section on one of arms and providing a light attenuation amount of −6 dB.
In any modulator of PTLs 1 and 2, since an optical signal is attenuated on one of arms, an optical loss is large, and the substrate becomes bigger caused by providing the optical power adjustment section.
Further, in the modulator described in PTL 1, due to a structure in which different types of substrates are optically connected, there is a case where an optical losing at a joint point occurs or a difference in coefficient of thermal expansion causes the breakage of modulator.